workflows/tests: run tap_syntax job only on Linux.

Some food for thought as:

- the only thing that is different on macOS is the `brew readall`
  behaviour on differing macOS/Linux Formula code (and that's more
  likely to be already working and will be caught by our self-hosted
  workers)
- the Linux workers are dramatically faster than the macOS ones and this
  saves the macOS Actions workers for the cases where they are actually
  needed.

.github/workflows/tests.yml

@@ -6,10 +6,7 @@ on:
 jobs:
   tap_syntax:
     if: github.repository == 'Homebrew/homebrew-core'
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        os: [ubuntu-latest, macOS-latest]
+    runs-on: ubuntu-latest
     steps:
     - name: Set up Homebrew
       id: set-up-homebrew